1. Field of the Invention
The present invention relates to an information recording medium, a reproduction apparatus reproducing information recorded on the medium, and a reproduction method.
2. Description of the Related Art
It has been known that minute structural changed areas having optical characteristics such as a refractive index to be different from those of the periphery thereof are formed when focusing short laser pulses inside a transparent medium such as fused silica, colorless crystal, or transparent ceramics. It is considered to three-dimensionally form a plurality of such minute structural changed areas inside a medium and to use the formed minute structural changed areas for recording information.
In JP-8-220688-A, U.S. Pat. No. 576,111, E. N. Glezer, et al., “Three-dimensional optical storage in side transparent materials”, Opt. Lett., Vol. 21, No. 24, pp. 2023-2025 (1996), Manabu Shiozawa et al., ISOM2012, Mo-D-01, Tokyo (2012), and Takao Watanabe et al., ISOM2012, Mo-D-02, Tokyo (2012), it has been disclosed to record data inside of a material, which is stable to be uniform, such as glass. Such a recording medium has an advantage of having superior durability and is suitable particularly for long-term storage of data. A mechanism for forming a local modification according to short laser pulses, for example, is described in M. Sakakura, et al., “Improved phase hologram design for generating symmetric light spots and its application for laser writing of waveguides”, Opt. Lett., Vol. 36, No. 7, pp. 1065-1067 (2011).
In addition, in Takao Watanabe et al., ISOM2012, Mo-D-02, Tokyo (2012), a reproduction method for using a photograph captured by using a camera attached to a microscope of about 20 magnifications has been disclosed. A result of the reproduction of digital data which is recorded in four layers by performing signal processing for improving the contrast of minute structural changed areas of the photograph with an error rate of a 10−3 base has been reported. In such a method, since the reproduction is performed using an image, unlike a general optical disc, high-speed rotation and precise tracking are not necessary. Accordingly, there are advantages of easily reproducing a read system even after the elapse of a long time and being capable of reading the data.
However, in the method disclosed in Takao Watanabe et al., ISOM2012, Mo-D-02, Tokyo (2012), since the microscope is used, the reproduction can be performed in a case where a technical expert operates in a laboratory having a vibration-proof function. However, it is difficult for a general user to perform the reproduction in a normal residential space. In order for a general user to perform the reproduction in a normal residential space, it is necessary to facilitate a composition rule, focusing, photographing timing, and the like. Conventionally, a review from this viewpoint has not been made. Accordingly, there has been no read system in which handling for reproducing minute structural changed areas included in a transparent medium as digital bit information is simple.
Meanwhile, currently, while a two-dimensional bar code that is generally distributed is one recording form of digital data having an error correction function, commonly, it is printed to have a relatively large size (a degree to which a black/white shape corresponding to one bit is visible) with high contrast on a sheet. Accordingly, the code can be reproduced by only holding a camera attached to a cellular phone or the like using the hand and photographing the sheet on which the code has been printed.
However, it is difficult to simply reproduce a two-dimensional bar code that is recoded as minute regions having low contrast in a transparent medium as described above or general digital data by using a camera that is attached to a cellular phone or the like.